Bond order is a concept from chemistry and thus you probably are best off digging out a corresponding text book. It also is a property that is computed during the calculation of the ReaxFF potential and a measure for the “order” of a bond. So you should also read up on the ReaxFF publications. The bond order cutoff in fix reaxff/species then is an empirical value determining when an interaction between two atoms is strong enough to be considered a bond. I cannot tell you what is a suitable cutoff value for your specific compound. That requires specific knowledge of the chemistry of your reactions.